Wellbores are formed in subterranean formations for various purposes including, for example, extraction of oil and gas from the subterranean formation and extraction of geothermal heat from the subterranean formation. Wellbores may be formed in a subterranean formation using various types of drill bits along with other wellbore tools. The downhole environment can be very harsh, with temperatures as high as 175° C. or more, and pressures as high as 100 MPa or more. Wellbore tools and the components thereof must be able to withstand such extreme pressures and temperatures. Furthermore, drilling of the subterranean formation generates extensive debris, which may be carried within the pressurized drilling fluid. Wellbore tools may be subjected to prolonged contact with such pressurized drilling fluid.
Many wellbore tools include components that need to be sealed from exposure to pressurized drilling fluid. Thus, seals are provided between adjacent components of such tools to prevent the pressurized drilling fluid from flowing between the adjacent components. In some tools, the adjacent components may be configured to move relative to one another. Seals between such components are referred to as “dynamic” seals, whereas seals between adjacent components that do not move relative to one another are referred to as “static” seals. The seal is established by providing a fluid-tight joint between the components that is intended to prevent migration of liquids and/or gases through the interface between the components. Wellbore tools commonly utilize various polymeric and elastomeric seals to provide both static and dynamic sealing between stationary and moving components, respectively.
Wellbore tools also utilize grease to provide lubricity between moving parts. Greases for wellbore tools should be able to withstand the harsh downhole environment throughout use of the tool, and without changing properties or characteristics. Greases for wellbore tools need to exhibit suitable rheological properties such as viscosity over the service periods for the tools. Greases for wellbore tools also should remain at least substantially chemically inert with respect to the wellbore fluids and various chemicals found in the downhole environment. A variety of grease compositions have been reported for use in wellbore tools. Such grease compositions often exhibit a relatively high viscosity, and typically comprise refined petroleum (hydrocarbon) oil or mineral oil, which provides the basic lubricity of the composition and may constitute about ¾ of the total grease composition. For example, U.S. Pat. No. 4,358,384, issued on Nov. 9, 1982 to Newcomb, describes a grease for a roller cone drill bit that is based on a petroleum derived, hydrocarbon oil and metal soap or metal complex thickener. U.S. Patent Publication No. 2011/0048809, published on Mar. 3, 2011 to Duckworth et al., describes a grease for use with a roller cone drill bit that includes: a base oil selected from the group consisting of a mineral oil, a synthetic oil, and combinations thereof; a metal sulfonate thickener; at least one solid additive selected from the group consisting of graphite, polytetrafluoroethylene, silica, inorganic sulfur-containing solids, inorganic phosphorus-containing solids, inorganic boron-containing solids, and any combinations thereof; and at least one oil-soluble additive selected from the group consisting of extreme pressure additives, oxidation inhibitors, wear and friction reducing additives, and polymer additives. U.S. Pat. No. 7,312,185, issued on Dec. 25, 2007 to Willey et al., discloses the use of high-viscosity poly-α-olefin (PAO) and other synthetic lubricants such as alkylated naphthalene to enhance the thermal stability of a grease.